Many live events shown on broadcast television, such as sporting events, use multiple cameras to simultaneously film parts of the event from multiple vantage points. For example, during a professional football game, various cameras film not only the main activity of the ball and nearby players, but also players anticipated to receive the ball, coaches, players on the sidelines, fans, etc. This allows a broadcast of the football game to switch between the various camera angles to focus on specific activity on and off the field as it occurs. Additionally, various camera angles aid in officiating the game, by allowing the referees to review a play from multiple angles to allow for determination of, for example, where exactly a player's feet hit the ground and whether the player was in or out of bounds.
During a live broadcast of a sporting event, camera views are switched dynamically, with the director, producer or other production staff directing which camera angles to highlight and when. Such a broadcast is seamless to the viewer, who simply observes the resultant video broadcast with the camera angles switching to focus on various aspects of the field. Audio of the event is typically overlaid with commentary provided in real time by one or more announcers, with both the audio and video portions synchronized to match the real-time broadcast.
In video production, a clapperboard is used to synchronize video with separately recorded audio. During post-production, the clap noise made by the clapperboard and the visual closing of the clapstick can be aligned in time so that the audio and the video synchronize. Enhancing audio and video special effects can also be added during post-production and the precise alignment of audio and video effects is crucial to a good viewing experience. This technique is of little value in synchronizing broadcast video of live events, however, due at least in part from the need to transmit the video in real-time